U.S. patent application number 10/356190 was filed with the patent office on 2003-07-31 for tunable laser and laser current source.
Invention is credited to Carney, Robert, Pontis, George D., Sprock, Douglas A..
Application Number | 20030142702 10/356190 |
Document ID | / |
Family ID | 25411983 |
Filed Date | 2003-07-31 |
United States Patent
Application |
20030142702 |
Kind Code |
A1 |
Pontis, George D. ; et
al. |
July 31, 2003 |
TUNABLE LASER AND LASER CURRENT SOURCE
Abstract
A laser system including a controller for monitoring and
controlling various functions of a laser assembly. The laser
controller may include a wavelength tuning circuit for adjusting
and locking the wavelength of the external cavity. To perform
various monitoring and control functions, the controller may
include circuitry for monitoring various parameters associated with
operation of the laser, such as temperature indicating signals
and/or signals from light detectors such as photodiodes.
Inventors: |
Pontis, George D.; (Redwood
City, CA) ; Sprock, Douglas A.; (San Jose, CA)
; Carney, Robert; (Belmont, CA) |
Correspondence
Address: |
Cory G. Claassen
BLAKELY, SOKOLOFF, TAYLOR & ZAFMAN LLP
Seventh Floor
12400 Wilshire Boulevard
Los Angeles
CA
90025-1026
US
|
Family ID: |
25411983 |
Appl. No.: |
10/356190 |
Filed: |
January 30, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10356190 |
Jan 30, 2003 |
|
|
|
09900108 |
Jul 6, 2001 |
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Current U.S.
Class: |
372/20 |
Current CPC
Class: |
H01S 5/0687 20130101;
H01S 5/02415 20130101; H01S 5/06837 20130101; H01S 5/06804
20130101; H01S 5/141 20130101 |
Class at
Publication: |
372/20 |
International
Class: |
H01S 003/10 |
Claims
What is claimed is:
1. A controller for a tunable laser comprising: a control circuit
for generating a control signal to control the tunable laser; and a
sensing circuit for sensing an attribute associated with operation
of the tunable laser; wherein the sensing circuit senses said
attribute synchronously with generation of said control signal by
said control circuit.
2. The controller as recited in claim 1 wherein the control signal
controls an external cavity length of said tunable laser.
3. The controller as recited in claim 1 wherein said control signal
is a pulse width modulated signal.
4. The controller as recited in claim 3 wherein said pulse width
modulated signal controls an external cavity length of said tunable
laser.
5. The controller as recited in claim 4 wherein said pulse width
modulated signal is provided to a temperature controller.
6. The controller as recited in claim 1 wherein said attribute
sensed by said sensing circuit is a voltage across a gain
medium.
7. The controller as recited in claim 1 wherein said attribute
sensed by said sensing circuit is a voltage indicative of a
temperature.
8. The controller as recited in claim 6 wherein said voltage varies
as a function of a resistance of a thermistor.
9. The controller as recited in claim 1 wherein said attribute
sensed by said sensing circuit is a signal generated by a light
detector.
10. The controller as recited in claim 9 wherein said light
detector is a photodiode.
11. The controller as recited in claim 1 wherein said control
circuit and said sensing circuit include circuitry embodied within
a programmable logic device.
12. A method for controlling a tunable laser comprising: generating
a control signal to control the tunable laser; and sensing an
attribute associated with operation of the tunable laser
synchronously with generation of said control signal.
13. The method as recited in claim 12 wherein said control signal
is a pulse-width modulated signal for controlling an external
cavity length of said tunable laser.
14. The method as recited in claim 12 wherein said attribute is a
voltage across a gain medium.
15. A controller for a tunable laser comprising: means for
generating a control signal to control the tunable laser; and means
for sensing an attribute associated with operation of the tunable
laser synchronously with generation of said control signal.
16. The controller as recited in claim 15 wherein said control
signal is a pulse-width modulated signal for controlling an
external cavity length of said tunable laser.
17. The controller as recited in claim 15 wherein said attribute is
a voltage across a gain medium.
18. A laser system comprising: a tunable laser; and a controller
coupled to the tunable laser, the controller including: a control
circuit for generating a control signal to control the tunable
laser; and a sensing circuit for sensing an attribute associated
with operation of the tunable laser; wherein the sensing circuit
senses said attribute synchronously with generation of said control
signal by said control circuit.
19. The laser system as recited in claim 18 wherein the control
signal controls an external cavity length of said tunable
laser.
20. The laser system as recited in claim 18 wherein said control
signal is a pulse width modulated signal.
21. The laser system as recited in claim 21 wherein said pulse
width modulated signal controls an external cavity length of said
tunable laser.
22. The laser system as recited in claim 21 wherein said pulse
width modulated signal is provided to a temperature controller.
23. The laser system as recited in claim 18 wherein said tunable
laser includes a gain medium, wherein said attribute sensed by said
sensing circuit is a voltage across said gain medium.
24. A temperature sensing circuit comprising: a plurality of
temperature-dependent resistive elements; a switching circuit for
selectively coupling any given one of the temperature-dependent
resistive elements in series with a fixed resistance between a
first reference voltage and a second reference voltage, whereby a
temperature dependent voltage is established at a node between the
any given one of the temperature-dependent resistive elements and
the fixed resistance; and a common measurement path for conveying
the temperature dependent voltage to a processing circuit.
25. The temperature sensing circuit as recited in claim 24 wherein
each of said plurality of temperature-dependent resistive element
is a thermistor.
26. The temperature sensing circuit as recited in claim 24 wherein
one of said plurality of temperature-dependent resistive elements
is placed in proximity to a gain medium for measuring a temperature
associated with said gain medium.
27. The temperature sensing circuit as recited in claim 24 wherein
one of said plurality of temperature-dependent resistive elements
is placed in proximity to a grid generator for sensing a
temperature associated with the grid generator.
28. The temperature sensing circuit as recited in claim 24 wherein
one of said plurality of temperature-dependent resistive elements
is placed in proximity to a cavity length actuator for sensing a
temperature associated with the cavity length actuator.
29. The temperature sensing circuit as recited in claim 24 wherein
said switching circuit includes one or more multiplexers.
30. A laser system comprising: a tunable laser; and a controller
including a processing circuit for controlling the tunable laser
and a temperature sensing circuit, wherein the temperature sensing
circuit includes: a plurality of temperature-dependent resistive
elements; a switching circuit for selectively coupling any given
one of the temperature-dependent resistive elements in series with
a fixed resistance between a first reference voltage and a second
reference voltage, whereby a temperature dependent voltage is
established at a node between the any given one of the
temperature-dependent resistive elements and the fixed resistance;
and a common measurement path for conveying the temperature
dependent voltage to the processing circuit.
31. The laser system as recited in claim 30 wherein each of said
plurality of temperature-dependent resistive element is a
thermistor.
32. The laser system as recited in claim 30 wherein one of said
plurality of temperature-dependent resistive elements is placed in
proximity to a gain medium for measuring a temperature associated
with said gain medium.
33. The laser system as recited in claim 30 wherein one of said
plurality of temperature-dependent resistive elements is placed in
proximity to a grid generator for sensing a temperature associated
with the grid generator.
34. The laser system as recited in claim 30 wherein one of said
plurality of temperature-dependent resistive elements is placed in
proximity to a cavity length actuator for sensing a temperature
associated with the cavity length actuator.
35. A control circuit for generating a modulating output signal for
driving an optical path length modulator in a tunable laser
comprising: waveform generation circuitry for generating an analog
signal; an amplifier circuit to receive the analog signal and
configured to produce an amplified analog signal; and a transformer
including a primary coil coupled to the amplifier circuit in a
push-pull configuration and a secondary coil for providing the
modulating output signal.
36. The control circuit as recited in claim 35 wherein the waveform
generation circuitry includes a programmable logic device for
generating a digital output signal.
37. The control circuit as recited in claim 36 wherein a signal
conversion circuit converts said digital signal to said analog
signal.
38. The control circuit as recited in claim 37 wherein the signal
conversion circuit comprises a low-pass filter.
39. The control circuit as recited in claim 38 wherein said digital
signal is a pulse width modulated signal.
40. The control circuit as recited in claim 35 wherein said
amplifier circuit includes a pair of operational amplifiers having
outputs coupled to source and sink current flowing through said
primary coil of said transformer.
41. A laser system comprising: a tunable laser including an optical
path length modulator; and a control circuit for generating a
modulating output signal for driving the optical path length
modulator, the control circuit including: waveform generation
circuitry for generating an analog signal; an amplifier circuit to
receive the analog signal and configured to produce an amplified
analog signal; and a transformer including a primary coil coupled
to the amplifier circuit in a push-pull configuration and a
secondary coil for providing the modulating output signal.
42. The laser system as recited in claim 41 wherein the waveform
generation circuitry includes a programmable logic device for
generating a digital output signal.
43. The laser system as recited in claim 42 wherein a signal
conversion circuit converts said digital signal to said analog
signal.
44. The laser system as recited in claim 43 wherein the signal
conversion circuit comprises a low-pass filter.
45. The laser system as recited in claim 44 wherein said digital
signal is a pulse width modulated signal.
46. The laser system as recited in claim 40 wherein said amplifier
circuit includes a pair of operational amplifiers having outputs
coupled to source and sink current flowing through said primary
coil of said transformer.
47. A laser current source comprising: a drive transistor having an
output for supplying current to a laser gain medium device and a
control terminal; a control circuit for receiving a signal to
control a level of current supplied to the laser gain medium
device; a power source; a resistor coupled between the power source
and the control terminal of the drive transistor; and a common
control terminal transistor, coupled between the control circuit
and the control terminal of the drive transistor.
48. The laser current source as recited in claim 47 wherein said
common control terminal transistor is a field effect transistor
coupled in a common gate configuration.
49. The laser current source as recited in claim 47 wherein said
control circuit includes a digital-to-analog converter.
50. The laser current source as recited in claim 47 further
comprising a switch coupled to divert said current to the laser
device.
51. The laser current source as recited in claim 50 wherein said
switch is a transistor.
52. The laser current source as recited in claim 50 further
comprising a microprocessor configured to program said level of
current supplied to the laser gain medium device.
53. The laser current source as recited in claim 52 wherein an
operation of said switch is independent of operations of said
microprocessor.
54. A laser system comprising: a tunable laser having a laser gain
medium device; and a laser current source including: a drive
transistor having an output for supplying current to the laser gain
medium device and a control terminal; a control circuit for
receiving a signal to control a level of current supplied to the
laser gain medium device; a power source; a resistor coupled
between the power source and the control terminal of the drive
transistor; and a common control terminal transistor, coupled
between the control circuit and the control terminal of the drive
transistor.
55. A laser control circuit for performing wavelength locking in a
tunable laser comprising: a pathlength adjuster circuit for
controlling a pathlength associated with said tunable laser; a
modulation generator for providing a modulation of said pathlength;
a detector configured to detect an attribute of said tunable laser
that is dependent upon said modulation and to generate data
indicative of said attribute; and a signal processing device
configured to perform a Fourier Transform upon said data to derive
an error signal for controlling said pathlength adjuster
circuit.
56. The laser control circuit as recited in claim 55 wherein the
signal processing device is configured to perform a Fast Fourier
Transform.
57. A method for performing wavelength locking in a tunable laser
comprising: setting a pathlength associated with said tunable
laser; providing a modulation of said pathlength; detecting an
attribute of said tunable laser that is dependent upon said
modulation; generating data indicative of said attribute;
performing a Fourier Transform upon said data to derive an error
signal; and controlling said pathlength depending upon said error
signal.
58. A laser control circuit for performing wavelength locking in a
tunable laser comprising: means for setting a pathlength associated
with said tunable laser; means for providing a modulation of said
pathlength; means for detecting an attribute of said tunable laser
that is dependent upon said modulation; means for generating data
indicative of said attribute; means for performing a Fourier
Transform upon said data to derive an error signal; and means for
controlling said pathlength depending upon said error signal.
59. A laser system comprising: a tunable laser assembly; a control
circuit for controlling operation of said tunable laser assembly;
and a network interface coupled to said control circuit and
configured to allow remote control of said operation of said
tunable laser through said network interface.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to laser systems and, more
particularly, to electronic controllers for controlling and
monitoring operations of a tunable laser, such as an external
cavity diode laser.
[0003] 2. Description of the Related Art
[0004] Tunable external cavity diode lasers (ECDLs) are widely used
in lightwave test-and-measurement equipment and are becoming
recognized as essential components for the rapidly expanding fields
of wavelength division multiplexed (WDM) optical voice and data
communications. The many applications within these fields represent
many different sets of performance specifications. However, the
following requirements are typical: small size of the
optomechanical assembly and control system;, servo control of the
wavelength; and controllable frequency modulation (FM) at audio
rates (e.g., 100 Hz to 30 kHz) in order to broaden the
linewidth.
[0005] To achieve desired control over the operation of external
cavity diode lasers, electronic controllers are typically provided
that implement various functionality. This functionality may
include a current source for providing current to the laser, locked
wavelength tuning functionality, a modulation source, and various
other functionality to precisely control and monitor operation of
the tunable laser. It is typically desirable that the electronic
controller allow for versatile control of the tunable laser with
reasonable efficiency and a relatively small form factor. It is
also typically desirable that electrical noise within the system
and its impact upon various measurement functions be minimized.
SUMMARY OF THE INVENTION
[0006] A laser control system for monitoring and controlling
various functions of a laser assembly is provided. In one
embodiment, the laser assembly comprises a tunable external cavity
laser. The laser controller may include a wavelength tuning circuit
for adjusting and locking the wavelength of the external cavity.
The tuning circuit may include a modulation signal generator for
providing a modulation signal to a selected transmission element
that causes a corresponding modulation of the optical path of the
laser external cavity. Wavelength locking may be achieved by
monitoring transmission characteristics that vary due to the slight
modulation of the optical path. Such transmission characteristics
may be monitored, for example, by detecting variations in the
voltage across a gain medium or variations in the intensity of
light associated with the laser external cavity. The tuning circuit
may include a signal processor such as a microprocessor that
performs a Fourier Transform, such as a Fast Fourier Transform,
upon data indicative of the transmission characteristics to thereby
generate an error signal for adjusting the length of the optical
path of the external cavity.
[0007] To perform various monitoring and control functions, the
controller may include circuitry for monitoring various parameters
associated with operation of the laser, such as temperature
indicating signals and/or signals from light detectors such as
photodiodes. The controller may additionally detect other
parameters, such as a voltage across a gain medium. In one
embodiment, the sensing of such parameters is performed
synchronously with the generation of various control signals for
controlling operation of the external cavity laser. The control
signals may include signals for adjusting the external cavity
pathlength and for generating a modulation signal. The control
signals may be in the form of pulse-width modulated signals, which
may be generated by a programmable logic device. In one embodiment,
temperature-dependent resistive elements such as thermistors may be
used to provide signals indicative of the temperature of various
components of the laser assembly. A switching circuit may be
employed to couple a selected temperature-dependent resistive
element to a common measurement path for detecting a temperature
associated with the selected temperature-dependent resistive
element. In yet a further embodiment, a control circuit for
generating a modulating output signal may include a transformer
including a primary coil coupled to an amplifier circuit in a
push-pull configuration. A laser current source may be provided
that includes a control circuit for controlling a level of current
supplied to a laser device through a drive transistor, and a common
gate or common base configured transistor coupled between the
control circuit and a control terminal of the drive transistor. The
laser controller may include a network interface to allow remote
control of the laser.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a block diagram illustrating various aspects of a
tunable external cavity laser.
[0009] FIGS. 2A-2C and FIGS. 3A-3C are diagrams illustrating pass
band relationships associated with an external cavity laser.
[0010] FIG. 4 is a functional block diagram illustrating aspects of
one embodiment of a laser controller.
[0011] FIG. 5 is a diagram illustrating a relationship of a
modulation signal with respect to detected voltage modulation
across a gain medium.
[0012] FIG. 6 is a hardware block diagram illustrating various
aspects of one embodiment of a laser controller.
[0013] FIG. 6A is a flow diagram illustrating one embodiment of an
algorithm for performing wavelength locking.
[0014] FIG. 7 is a circuit diagram illustrating one embodiment of a
laser current source.
[0015] FIG. 8 is a circuit diagram illustrating one embodiment of
an analog interface.
[0016] FIG. 9 is flow diagram illustrating a method for performing
temperature measurements.
[0017] FIG. 10 is a circuit diagram illustrating one embodiment of
an amplifier circuit for generating a modulation signal.
[0018] While the invention is susceptible to various modifications
and alternative forms, specific embodiments are shown by way of
example in the drawings and are herein described in detail. It
should be understood, however, that drawings and detailed
description thereto are not intended to limit the invention to the
particular form disclosed, but on the contrary, the invention is to
cover all modifications, equivalents and alternatives falling
within the spirit and scope of the present invention as defined by
the appended claims.
DETAILED DESCRIPTION
[0019] The term "external cavity laser" as used herein is meant to
encompass any laser device wherein at least one external reflective
element is used to introduce optical feedback into a gain medium.
"External reflective element" means a reflective element that is
not actually a part of, or integral to, the gain medium itself.
[0020] FIG. 1 illustrates various aspects of one embodiment of a
tunable external cavity laser apparatus 10. The apparatus 10
includes a gain medium 12 and an end or external reflective element
14. Gain medium 12 may comprise a conventional Fabry-Perot diode
emitter chip having an anti-reflection (AR) coated rear facet 16
and a partially reflective front facet 18. Front facet 18 and end
mirror 14 define an external cavity for the apparatus 10. Gain
medium 12 emits a coherent beam from rear facet 16 that is
collimated by lens 20 to define an optical path 22 which is
co-linear with the optical axis of the external cavity. Rear and
front facets 16, 18 of gain medium 12 are aligned with the optical
axis of the external cavity as well. Light reflected from end
mirror 14 is fed back along optical path 22 into gain medium 12.
Conventional output coupler optics (not shown) may be associated
with front facet 18 for coupling the output of external cavity
laser 10 into an optical fiber (also not shown).
[0021] Transmission characteristics of the external cavity can be
probed or evaluated by monitoring the voltage across gain medium
12. In this regard, first and second electrodes 24, 26 may be
positioned adjacent to and operatively coupled to gain medium 12.
First electrode 24 is operatively coupled to a controller 48 via a
conductor 28, where the voltage across gain medium 12 may be
detected. A second electrode 26 is grounded via conductor 30. It is
noted that in various other embodiments, transmission
characteristics of the external cavity may be alternatively
evaluated by monitoring the output of one or more photodiodes 15
(or any other type of light detector) that may be positioned to
receive portions of light associated with the external cavity
laser. For example, in one embodiment, a photodiode may be
positioned to detect light propagating through the partially
reflective end mirror 14. In another embodiment, a photodiode may
be positioned to detect light propagating through front facet 18 of
gain medium 12. Photodiodes may be positioned to detect light
associated with the external cavity laser at other particular
locations, as desired.
[0022] Error signals may be derived from the voltage measured
across gain medium 12 to correct or otherwise adjust the
transmission characteristics associated with the external cavity.
Details regarding specific implementations of this functionality
are provided further below.
[0023] Other transmission elements associated with the external
cavity may include a grid generator element and a channel selector
element, which are respectively shown in FIG. 1 as a grid etalon 34
and a wedge etalon 36 positioned in optical path 22 between gain
medium 12 and end mirror 14. Grid etalon 34 typically is positioned
in optical path 22 before wedge etalon 26. Grid etalon 34 operates
as an interference filter, and the refractive index and optical
thickness of grid etalon 34 give rise to a multiplicity of minima
within the communication band at wavelengths which coincide with
the center wavelengths of a selected wavelength grid which may
comprise, for example, the ITU (International Telecommunications
Union) grid. Other wavelength grids may alternatively be selected.
Grid etalon 34 thus has a free spectral range (FSR) which
corresponds to the spacing between the gridlines of the ITU grid or
other selected grid, and the grid etalon 34 thus operates to
provide a plurality of pass bands centered on each of the gridlines
of the wavelength grid. Grid etalon 34 has a finesse (free spectral
range divided by full width half maximum or FWHM) which suppresses
neighboring modes of the external cavity laser between each channel
of the wavelength grid.
[0024] Grid etalon 34 may be a parallel plate solid, liquid or gas
spaced etalon, and may be tuned by precise dimensioning of the
optical thickness between its faces by thermal expansion and
contraction via temperature control. The grid etalon 34 may
alternatively be tuned by tilting to vary the optical thickness
between faces 38, 40, or by application of an electric field to an
electro-optic etalon material. Various other grid generating
elements are known to those skilled in the art and may be used
place of grid etalon 34. Grid etalon 34 may be thermally controlled
using a temperature controller (TEC) 66 to prevent variation in the
selected grid which may arise due to thermal fluctuation during
operation of external cavity laser 10. Grid etalon 34 alternatively
may be actively tuned during laser operation.
[0025] Wedge etalon 36, like grid etalon 34, acts as an
interference filter, but with nonparallel reflective faces 42, 44
providing a tapered shape. Wedge etalon 36 may comprise a tapered
transparent substrate, a tapered air gap between the reflective
surfaces of adjacent transparent substrates, or a thin film wedge
interference filter as described further below.
[0026] The relative size, shape and distances between the various
optical components of external cavity laser 10 are in some
instances exaggerated for clarity and are not necessarily shown to
scale. External cavity laser 10 may include additional transmission
elements (not shown), such as focusing and collimating components,
and polarizing optics configured to remove spurious feedback
associated with the various components of external cavity laser 10.
The location of grid generator 34 and channel selector 36 may vary
from that shown in FIG. 1.
[0027] Wedge etalon 36 defines a plurality of pass bands which are
substantially broader than the pass bands of the grid etalon 34,
with the broader pass bands of the wedge etalon 36 having a
periodicity substantially corresponding to the separation between
the shortest and longest wavelength channels defined by the grid
etalon 34. In other words, the free spectral range of the wedge
etalon 36 corresponds to the full wavelength range of the
wavelength grid defined by grid etalon 34. The wedge etalon 36 has
a finesse which suppresses channels adjacent to a particular
selected channel.
[0028] The wedge etalon 36 may be used to select between multiple
communication channels by changing the optical thickness between
faces 42, 44 of wedge etalon 36. This is achieved by translating or
driving wedge etalon 36 in a direction parallel to the taper of
wedge etalon 36 and perpendicular to optical path 22 and the
optical axis of external cavity laser 10. Each of the pass bands
defined by the wedge etalon 36 supports a selectable channel, and
as the wedge is advanced or translated into optical path 22, the
beam traveling along optical path 22 passes through increasingly
thicker portions of wedge etalon 36 which support constructive
interference between opposing faces 42, 44 at longer wavelength
channels. As wedge etalon 36 is withdrawn from optical path 22, the
beam will experience increasingly thinner portions of wedge etalon
36 and expose pass bands to the optical path 22 which support
correspondingly shorter wavelength channels. The free spectral
range of wedge etalon 36 corresponds to the complete wavelength
range of grid etalon 34 as noted above, so that a single loss
minimum within the communications band can be tuned across the
wavelength grid. The combined feedback to gain medium 12 from the
grid etalon 34 and wedge etalon 36 support lasing at the center
wavelength of a selected channel. Across the tuning range, the free
spectral range of the wedge etalon 36 is broader than that of grid
etalon 34.
[0029] Wedge etalon 36 is positionally tuned via a tuning assembly
which comprises a drive element 46 structured and configured to
adjustably position wedge etalon 36 according to selected channels.
Drive element 46 may comprise, for example, a stepper motor
together with suitable hardware for precision translation of wedge
etalon 36. Drive element 46 may alternatively comprise various
types of actuators, including, but not limited to, DC servomotors,
solenoids, voice coil actuators, piezoelectric actuators,
ultrasonic drivers, shape memory devices, and like linear
actuators.
[0030] Drive element 46 is operatively coupled to controller 48
which provides signals to control the positioning of wedge etalon
36 by way of drive element 46. Controller 48 may include a data
processor and memory (not shown in FIG. 1) having lookup tables of
positional information for wedge etalon 36 which correspond to
selectable channel wavelengths.
[0031] When external cavity laser 10 is tuned to change from one
communication channel to another, controller 48 may convey signals
to drive element 46 according to positional data in a lookup table,
and drive element 46 translates or drives wedge etalon 36 to a
position wherein the optical thickness of the portion of the wedge
etalon 36 positioned in optical path 22 provides constructive
interference which supports the selected channel. A position
detector 50 such as a linear encoder may be used in association
with wedge etalon 36 and drive element 46 to ensure correct
positioning of wedge etalon 36 by driver 46. Alternatively, a
single point position electro-optic detector may be provided to
locate a "home" position associated with wedge etalon 36 during
initialization of the system.
[0032] An electro-optically activated modulation element 58 is also
shown positioned in optical path 22 before end mirror 14. In the
embodiment of FIG. 1, end mirror 14 is formed as a reflective
coating directly on the electro-optic material of modulation
element 58. Thus, the end mirror 14 and modulation element 58 are
combined into a single component. In other embodiments, end mirror
14 may be formed on an element that is separate from modulation
element 58. Details regarding the function of modulation element 58
will be provided further below.
[0033] The pass band relationship of the grid etalon 34, wedge
etalon 36 and the external cavity defined by front facet 18 and end
mirror 14 are illustrated graphically in FIG. 2A through FIG. 2C,
which show external cavity pass bands PB1, grid etalon pass bands
PB2, and wedge etalon pass bands PB3. Relative gain is shown on the
vertical axis and wavelength on the horizontal axis. As can be
seen, free spectral range of the wedge etalon 36 (FSR.sub.Channel
Sel) is greater than the free spectral range of the grid etalon 34
(FSR.sub.Grid Gen), which in turn is greater than the free spectral
range of the external cavity (FSR.sub.cavity). The band pass peaks
PB1 of the external cavity periodically align with the center
wavelengths of pass bands PB2 defined by the wavelength grid of
grid etalon 34. There is one pass band peak PB3 from the wedge
etalon 36 which extends over all of the pass bands PB2 of the
wavelength grid. In the specific example shown in FIGS. 2A-2C, the
wavelength grid extends over sixty four channels spaced apart by
one half nanometer (nm) or 62 GHz, with the shortest wavelength
channel at 1532 nm, and the longest wavelength channel at 1563.5
nm.
[0034] The finesse of grid etalon 34 and wedge etalon 36 determine
the attenuation of neighboring modes or channels. As noted above,
finesse is equal to the free spectral range over the full width
half maximum, or finesse=FSR/FWM. The width for a grid etalon pass
band PB2 at half maximum is shown in FIG. 2B, and the width for a
wedge etalon pass band PB3 at half maximum is shown in FIG. 2C. The
positioning of grid etalon 34 and wedge etalon 36 within the
external cavity improves side mode suppression.
[0035] The tuning of the band pass PB3 of wedge etalon 36 between a
channel centered at 1549.5 nm and an adjacent channel at 1550 nm is
illustrated graphically in FIGS. 3A-3C, wherein the selection of a
channel generated by grid etalon 24 and the attenuation of adjacent
channels or modes is shown. The external cavity pass bands PB1
shown in FIGS. 2A-2C are omitted from FIGS. 3A-3C for clarity. The
grid etalon 34 selects periodic longitudinal modes of the external
cavity corresponding to the grid channel spacing while rejecting
neighboring modes. The wedge etalon 36 selects a particular channel
in the wavelength grid and rejects all other channels. The selected
channel or lasing mode is stationary at one particular channel for
filter offsets in the range of approximately plus or minus one half
channel spacing. For larger channel offsets the lasing mode jumps
to the next adjacent channel.
[0036] In FIG. 3A, the wedge etalon pass band PB3 is centered with
respect to the grid channel at 1549.5 nm. The relative gain
associated with pass band PB2 at 1549.5 nm is high, while the
relative gain levels associated with adjacent pass bands PB2 at
1549.0 nm and 1550.0 nm are suppressed relative to the selected
1549.5 nm channel. The gain associated with pass bands PB2 at
1550.5 nm and 1548.5 nm is further suppressed. The dashed line
indicates the relative gain for pass bands PB2 without suppression
by wedge etalon 36.
[0037] FIG. 3B shows the wedge etalon pass band PB at a position in
between the channels at 1549.5 nm and 1550.0 nm, as occurs during
channel switching. The relative gain associated with pass bands PB2
at 1549.5 nm and 1550.0 are both high, with neither channel
suppressed. The relative gain levels associated with pass bands PB2
at 1549.0 nm and 1550.5 nm are suppressed relative to the 1549.5 nm
and 1550.0 nm channels. The dashed line indicates the relative gain
for pass bands PB2 without suppression by wedge etalon 36.
[0038] FIG. 3C shows the wedge etalon pass band PB3 centered with
respect to the grid channel at 1550.0 nm, with the relative gain
associated with the pass band PB2 at 1550.0 nm being high, while
the relative gain levels associated with adjacent pass bands PB2 at
1549.5 nm and 1550.5 nm are suppressed relative to the selected
1550.0 nm channel, and the gain associated with pass bands PB2 at
1551.0 nm and 1549.0 nm is further suppressed. Again, the dashed
line indicates the relative gain for pass bands PB2 without
suppression by wedge etalon 36.
[0039] As can be seen from FIG. 2 and FIG. 3, non-optimal
positioning or tuning of channel selector 36, end mirror 14 and/or
grid generator 34 will result in mis-alignment of pass bands PB1,
PB2 and PB3 and give rise to attenuation in optical output power
from the external cavity laser 10. Monitoring of voltage across
gain medium 12 via voltage sensor 32 allows such external cavity
transmission characteristics to be probed or evaluated during laser
operation. Error signals derived from the monitored voltage can
then be used to adjust or servo the transmission profile of the
external cavity during operation such that pass bands PB1, PB2 and
PB3 are optimally aligned with respect to each other by suitable
repositioning or adjusting of end mirror 14, wedge etalon 36 and/or
grid etalon 34, thus providing accurate wavelength tuning and
stability. As stated previously, in various other embodiments, such
error signals may be alternatively derived using one or more
photodiodes 15.
[0040] The electro-optic modulation element 58 as shown in FIG. 1
provides a signal modulation in the form of a frequency dither,
which may be introduced into the optical path length of the
external cavity laser 56 by the presence of element 58 in optical
path 22. Modulation element 58 may comprise an etalon of
electro-optic material such as lithium niobate, and has a
voltage-adjustable refractive index. The signal modulation may
comprise, for example, a frequency modulation of about 20 KHz.
Adjustment of voltage across the electro-optic material of tuning
element changes the effective optical thickness of modulation
element 58, and hence the overall optical path length l across the
external cavity (between diode facet 18 and end mirror 14) of
external cavity laser 56. Thus, electro-optic modulation element 58
may provide both (i) a frequency modulation signal or dither to the
external cavity, and (ii) a mechanism to tune or adjust the
external cavity optical path length by voltage applied across
modulation element 58. Electro-optic modulation element 58 may
alternatively comprise an acousto-optic device, mechanical device,
or other device capable of introducing a detectable frequency
dither or modulation signal into the output of the external
cavity.
[0041] Modulation of the optical path length l via frequency dither
introduced by element 58 produces intensity variations in the
output power of external cavity laser 56 which are detectable in
the monitored voltage across gain medium 12, due to optical
feedback thereinto from the external cavity. These intensity
variations will decrease in magnitude and phase error as a laser
cavity mode is aligned with the center wavelength of the pass bands
defined by grid generator 34 and channel selector 36. In other
words, the intensity variations and phase error in the modulation
signal are minimal or nominally zero when pass bands PB1, PB2 and
PB3 are optimally aligned as shown in FIGS. 2A-2C. The use of
intensity variation and phase error in the modulated signal with
respect to error signal determination is described further below
with reference to FIG. 5.
[0042] A tuning arm 74 may further be employed to positionally
adjust end mirror according to input from controller 48. Tuning arm
74 may be made from a material having a high coefficient of thermal
expansion, such as aluminum or other metal or metal alloy.
Controller 48 is operatively coupled to a thermoelectric controller
78 via line 80. Thermoelectric controller 78 is coupled to tuning
arm and is configured to adjust the temperature of arm 74. Thermal
control (heating or cooling) of tuning arm 74, according to signals
from controller 76, may be used in this embodiment to control the
position of end mirror 14 and the length of optical path l of the
external cavity defined by end mirror and front facet 18 of gain
medium 12 in an optimal position.
[0043] The frequency modulation introduced by modulation element 58
is detectable by controller 48 by monitoring the voltage across the
gain medium 12 or a signal from one or more photodiodes 15, and the
frequency modulation includes variations in magnitude and phase
error indicative of laser cavity mode alignment with the center
wavelength of the pass bands defined by grid generator 34 and
channel selector 36, as noted above. Controller 48 maybe configured
to derive an error signal from the modulation introduced: by the
frequency dither, and to communicate a compensation signal to
thermoelectric controller 78, which accordingly heats or cools
tuning arm 74 to position end mirror 14 and adjust the optical path
length l of external cavity laser to null out the error signal.
[0044] FIG. 4 is a functional block diagram illustrating aspects of
one embodiment of controller 48. The controller of FIG. 4 includes
a tuning circuit 84, a current source 86 operatively coupled to
gain medium 12 via line 28, a grid controller 88 operatively
coupled to thermoelectric controller 66 via line 68, and a channel
controller 90 operatively coupled to drive element 46 via line 70.
The current source 86 controls the power delivered to gain medium
12. The grid controller 88 maintains the referential integrity of
grid etalon 34 by thermal control thereof using thermoelectric
controller 66 to heat or cool grid etalon 34 as required. Channel
controller 90 directs drive element 46 to position or otherwise
adjust channel selector 36 for selection of desired transmission
bands in the grid defined by grid etalon 34.
[0045] Tuning circuit 84 comprises a signal processor 94, a voltage
detector 96, a path length adjuster 100, and a modulation signal
generator 102. Modulation signal generator 102 provides a frequency
dither or modulation signal to a selected loss element (e.g.,
modulation element 58) that causes a corresponding modulation of
the optical path l of the laser external cavity. The modulation
frequency and amplitude may be selected, for example, to increase
effective coupling efficiency. The voltage across gain medium 12
(or a signal derived from a photodiode 15, as discussed previously)
may be detected by voltage detector 96 and communicated to signal
processing circuit 94. The signal processing circuit 94 may be
configured to determine the alignment of passbands PB1 (FIG. 2 and
FIG. 3) of the external cavity with passbands PB2 of grid etalon 34
and passbands PB3 of channel selector 36, and to generate
corresponding error information.
[0046] Pathlength adjuster 100 generates an error correction or
compensation signal, from the error information provided by signal
processing 94, that is used to adjust the optical path length l of
the external cavity in order to optimize the relationship between
the modulation signal and the intensity signal. When an external
cavity mode or pass band PB1 is aligned with bands PB2 and PB3
generated by grid generator 34 and channel selector 36, intensity
variations at the modulation frequency (and odd multiples thereof)
in the coherent beam traveling optical path 22 are substantially
minimized, as discussed further below with reference to FIG. 5.
Concurrently, the voltage signal intensity will vary at twice the
modulation frequency. Either or both of these detectable effects
are usable to evaluate external cavity loss associated with loss
characteristics associated with the positioning or
inter-relationship of end mirror 14, grid generator 34 and channel
selector 36, and to generate error signals usable for adjustment of
cavity loss characteristics such that the modulation signal and
intensity signal are optimized. As discussed previously, in one
embodiment, adjustment of the optical path length l may be carried
out via thermal positioning of end mirror 14 in conjunction with
tuning arm 74 and temperature controller 78. In other embodiments,
pathlength adjuster 100 may control other elements that adjust the
optical pathlength l of the external cavity laser.
[0047] Referring now to FIG. 5, the relationship of the dither
modulation signal introduced to an external cavity with respect to
the detected voltage modulation across gain medium 12 is
illustrated graphically as wavelength versus relative intensity.
FIG. 2 shows a grid etalon pass band PB2, together with frequency
or dither modulation signals 104A, 104B, 104C corresponding to
external cavity laser modes 106A, 106D and 106C respectively.
Frequency modulation signals 104A-C are introduced to the laser
external cavity by voltage modulation of electro-optic element 58
in the manner described above. As shown in FIG. 6, laser mode 106A
is off-center with respect to the center of pass band PB2 towards
the shorter wavelength side of pass band PB2, while laser mode 106B
is located at about the center wavelength of pass band PB2, and
laser mode 106C is located on the longer wavelength side of pass
band PB2. Laser mode wavelength 106B corresponds to a wavelength
lock position and represents an optimal loss profile for the
external cavity. Laser modes 106A and 106B are off-center with
respect to pass band PB2 and result in non-optimal cavity loss
profiles which will require adjustment of the external cavity
length l, either by adjusting the effective optical thickness of
electro-optic element 58 or by positioning end mirror 14 as
described above.
[0048] The voltage detected across gain medium 12 by voltage
detector 96 for dither signals 104A, 104B and 104C are shown
respectively as voltage modulation signals 108A, 108B and 108C on
the right side of FIG. 6, which correspond respectively to the
laser mode wavelengths 106A, 106B and 106C. The location of laser
mode 106A at a wavelength shorter than that of the center
wavelength of pass band PB2 results in voltage signal 108A having a
modulation that is in phase with the dither modulation signal;
104A. The location of laser mode 106C at a greater wavelength than
the center wavelength of pass band PB2 results in a modulation of
voltage signal 108C that is out of phase with respect to the
modulation of dither signal 104C.
[0049] The location of each laser mode wavelength with respect to
the slope of pass band PB2 affects the amplitude of the
corresponding voltage signal. Thus, voltage signal 108A, which
corresponds to laser mode 106A wavelength on a relatively steep
slope of pass band PB2, has a relatively large modulation
amplitude, while voltage signal 108C, which corresponds to laser
mode 106C associated with a portion of pass band PB2 having a less
steep slope, has a correspondingly smaller modulation amplitude.
Voltage signal 108B, which corresponds to centered laser mode 106B,
has a minimal modulation amplitude since the period of the dither
modulation signal 104B occurs symmetrically about the center
wavelength of pass band PB2. The frequency of the dominant
intensity in the case of voltage signal 108B in this instance is
twice the frequency of dither modulation signal 104B.
[0050] From FIG. 5 it can be seen that the amplitude of the
modulation detected in the voltage across gain medium 12 indicates
the magnitude of correction or adjustment required for the laser
external cavity, while the phase of voltage signal modulation
indicates the direction of the adjustment. The amplitude of dither
modulation signals 104A-C is selected so that, during wavelength
lock, the variation in the intensity of voltage signal modulation
is held to acceptable levels for the particular use of the external
cavity laser. The frequency of the dither modulation is chosen to
be high enough to provide coherence control, but low enough to
prevent interference with information modulated onto the carrier
signal provided by the external cavity laser during
transmission.
[0051] FIG. 6 is a hardware block diagram illustrating various
aspects of one embodiment of a laser controller that may be
configured to implement the functionality of the control system as
depicted in FIG. 4. Various features of a laser assembly such as
the assembly 10 described previously in conjunction with FIG. 1 are
also illustrated in FIG. 6. Features that correspond to those of
FIG. 1 and FIG. 4 are numbered identically for simplicity and
clarity. It is noted that in other embodiments, various features of
the laser controller of FIG. 6 as discussed below may be used in
conjunction with other configurations of laser assemblies.
Furthermore, such controllers and laser assemblies may omit various
functionality as discussed above in conjunction with FIGS. 1-5.
[0052] The laser controller of FIG. 6 includes a microprocessor
(CPU) 602 coupled through an interconnect bus 610 to a read-only
memory (ROM) 604, a random access memory (RAM) 606 and a field
programmable gate array (FPGA) 608. FPGA 608 is coupled to a
stepper motor driver 612, amplifiers 614-616, and a low pass filter
618. FPGA 608 is further shown coupled to a digital-to-analog
converter 620, an analog interface unit 622, and an
analog-to-digital converter 624. A laser current source 86 is shown
coupled to an output of digital-to-analog converter 620.
[0053] Power to the components of the laser controller illustrated
in FIG. 6 is provided by a power source 630. In one embodiment,
power source 630 receives 5 volt input power and generates output
power of varying voltage levels to appropriately supply power to
the components of controller 600. Power source 630 may be
implemented using a high efficiency switching regulator
circuit.
[0054] Microprocessor 602 and FPGA 608 operate concurrently and in
cooperation with each other to perform various functionality as
depicted in FIG. 4 and described hereinbelow. It is noted that
operations performed by microprocessor 602 may be conducted in
accordance with the execution of software code stored within ROM
604. In one embodiment, microprocessor 602 is implemented using a
general purpose microprocessor, such as a Motorola MCF5206e
microprocessor. It is noted that in other embodiments, a digital
signal processor or other specialized hardware may be employed in
place of microprocessor 602. It is further noted that in other
embodiments, other programmable logic devices, such as a CPLD
(Complex Programmable Logic Device) may be employed in the place of
FPGA 608. Alternatively, one or more ASICs (Application Specific
Integrated Circuits) could be employed. Still additional
embodiments are contemplated that combine various functionality of
microprocessor 602 and FPGA 608 as described herein within a single
device.
[0055] Generally speaking, microprocessor 602 and FPGA 608
collectively operate to measure and process various parameters
associated with the operation of laser assembly 10 and to perform
various control functions. In one particular implementation,
microprocessor 602 and FPGA 608 are clocked at 40 MHz.
[0056] As illustrated in FIG. 6, laser assembly 10 may include a
laser temperature sensor 631 located in proximity to gain medium
12, a grid generator temperature sensor 632 located in proximity to
grid etalon 34, a cavity length actuator temperature sensor 633
located in proximity to tuning arm 74, and an ambient temperature
sensor 634. Each of the sensors 631-634 may be implemented using a
thermistor, although other temperature dependent devices may be
employed in other embodiments. Laser assembly 10 may further
include one or more photodiodes 15 positioned at selected locations
of the laser assembly to receive light associated with the
operation of the external cavity laser. In the illustrated
embodiment, FPGA 608 may be programmed to periodically detect
signals associated with each of sensors 631-634, photodiodes 15,
and/or gain medium 12 through analog interface 622 and
analog-to-digital converter 624. For this purpose, analog interface
622 includes multiplexers 650-652 and an anti-alias filter 653.
Multiplexers 650-652 operate under the control of FPGA 608 to
periodically couple a signal associated with a selected one of
sensors 631-634, split detector 658, or gain medium 12 for signal
detection. These operations will be described in further detail
below.
[0057] FPGA 608 is additionally configured to generate control
signals for controlling various functionality of laser assembly 10.
More particularly, in the embodiment of FIG. 6, FPGA 608 is
configured to generate a control signal for controlling the
position of a channel selector stepper motor 46a through a stepper
motor driver 612 (which are collectively representative of the
drive element 46 of FIG. 1). As discussed previously, a position
indicator 50 coupled to FPGA 608 may further provide an indication
of the position of drive element 46 (or to indicate when the drive
element is at a home position). The control signal generated by
FPGA 608 for controlling the position of stepper motor 46A may be
driven in accordance with a control value stored within a storage
location of FPGA 608. This storage location may be periodically
updated with new values through the execution of instructions
executed by microprocessor 602.
[0058] FPGA 608 may also be configured to generate a control signal
for controlling grid generator temperature controller (TEC) 66,
which regulates the temperature of grid etalon 34. FPGA 608 may
similarly generate control signals for controlling a cavity length
actuator temperature controller 78, which regulates the temperature
of tuning arm 74, and a laser 79, which regulates the temperature
of gain medium 12. In one embodiment, each of the temperature
controllers 66, 78 and 79 are controlled by pulse-width modulated
(PWM) signals generated by FPGA 608. Each of the temperature
controllers may be implemented using a peltier device. In one
specific implementation, the pulse-width modulated signals are
generated at a repetition rate of 200 kHz. Amplifiers 614-616 are
provided to amplify the PWM signals generated by FPGA 608. It is
noted that in alternative embodiments, other forms of control
signals may be generated to control selected functions of laser
assembly 10.
[0059] FPGA 608 may further be configured to generate a modulation
signal for driving modulation element 58. For this purpose, FPGA
608 may be configured to generate a pulse-width modulated signal
which is input to a low-pass filter 618 which correspondingly
provides an analog modulation signal that is passed to an amplifier
619. In one particular implementation, the modulation signal
provided from the output of low-pass filter 618 is in the form of a
sinusoidal wave at 20 kHz. The PWM signal generated by FPGA 608 may
have a frequency consistent with that of the other PWM signals
generated by FPGA 608. For example, in one embodiment, the PWM
signal has a frequency of 200 KHz. Further details regarding
generation of a modulation signal for driving modulation element 58
will be provided further below.
[0060] The sampling of signals associated with sensors 631-634,
photodiodes 15, and/or gain medium 12 may be performed
synchronously with the generation of the PWM control signals that
drive temperature controllers 66, 78, and 79, as well as the PWM
signal provided to low pass filter 618. The precise timing and
synchronization of the control signals with the detected signals
reduces the potential noise sources to a DC offset by mixing the
fundamental component down to 0 or DC. The DC offsets can be
subtracted from the signal in interest.
[0061] As described previously in conjunction with FIG. 5, the
amplitude of the modulation detected in the voltage cross gain
medium 12 indicates the magnitude of correction or adjustment
required for the laser external cavity, while the phase of voltage
signal modulation indicates the direction of the adjustment.
Accordingly, in one embodiment the voltage across laser 12 is
periodically measured by FPGA 608 through analog interface 622 and
analog-to-digital converter 624. The voltage signal may be
amplified with a single stage pre-amp within analog interface 622
and then multiplexed through multiplexer 650 into a common
anti-alias filter 653. Multiplexer 652 is set to provide the output
of anti-alias filter 653 to analog-to-digital converter 624.
[0062] In one particular implementation, following a predetermined
settling time after FPGA 608 sets multiplexers 650 and 652 in a
manner to convey a signal corresponding to the voltage across gain
medium 12 to analog-to-digital converter 624, FPGA 608 performs a
burst of, for example, 50 separate and consecutive voltage readings
associated with the voltage across gain medium 12. Each of the
voltage readings (in the form of digital data generated by
analog-to-digital converter 624) may be temporarily stored within
FPGA 608, and is subsequently transferred into RAM 606. Upon
receipt of data from analog-to-digital converter 624 by FPGA 608,
FPGA 608 may signal microprocessor 602 which may responsively
invoke an internal direct memory access control mechanism to carry
out the transfer of the data from FPGA 608 to RAM 606.
[0063] Upon storing a set of data indicative of the voltage across
gain medium 12 within RAM 606, microprocessor 602 performs a
Fourier Transform to transform the temporal data to a frequency
domain to separate the DC, fundamental and/or harmonic terms. In
one embodiment, microprocessor 602 executes a Fast Fourier
Transform (FFT) routine. The FFT routine may be optimized for
integer input data as supplied from analog-to-digital converter
624, and may be configured to compute only the output terms of
particular interest, such as the fundamental component. As
discussed previously, by calculating, for example, the magnitude
and phase of the fundamental component, an error signal may be
generated to adjust the cavity length. Thus, upon calculation of
the error signal, microprocessor 602 writes a value derived from
the error signal to a location within FPGA 608 which controls the
pulse width of the PWM signal provided to amplifier 616 to drive
cavity length actuator temperature controller 78. It is noted that
in other embodiments, the error signal may be used to control other
mechanisms within a laser assembly to adjust cavity length. It is
also noted that in other embodiments, similar measurements may
alternatively be taken from one or more photodiodes 15 (or other
light detectors) to derive the error signal. In various embodiments
and depending upon the signals of interest, multiplexer 650 and/or
anti-alias filter 653 of analog interface 622 may be omitted.
[0064] FIG. 6A illustrates one embodiment of an algorithm for
performing wavelength-locking. The wavelength-locking algorithm as
depicted in FIG. 6A may be implemented by code executed within
microprocessor 602, and in conjunction with the control of FPGA 608
as described herein. When initiated, the algorithm begins by
computing offsets, initializing variables, and placing the cavity
length actuator 78 in an initial starting position (steps 670 and
671). Next, the algorithm enters a locking loop where the cavity
length actuator sensor 633 is measured and the quality of the
locking is determined. The quality of locking may be determined by
computing a decaying integral of the error signal. If the cavity
length actuator sensor indicates a temperature within a
predetermined range and if the lock quality is sufficient as
determined during step 672, modulation data is acquired during step
673. As discussed previously, the modulation data may be in the
form of a set of readings associated with the voltage across gain
medium 12, or may be associated with a set of readings taken from
one or more photodiodes 15. The fundamental modulation component of
the most recent gain medium potential measurement may be used to
compute cavity length errors (step 674) and is applied to a
compensator in order to minimize the fundamental component. As
stated previously, the fundamental modulation component may be
computed by an FFT routine executed by microprocessor 602. In other
embodiments, other harmonics of the gain medium voltage or
photodiode currents may alternatively or additionally be determined
and used to compute the error signal. The slew rate associated with
the error signal may be limited during step 675. During step 676,
microprocessor 602 may write a value in a corresponding storage
location of FPGA 608 that controls the generation of the PWM signal
to cavity length actuator temperature controller 78 to thereby
cause corrections to the cavity length to be made. The locking
algorithm repeats these steps unless the cavity length actuator
sensor 633 indicates a temperature that is out of a predetermined
range or if the lock quality is poor (step 672). An integral error
term of the error signal may be reset during step 676, and the
cavity length actuator (e.g., tuning arm 74) may be returned to the
initial starting position during step 677. The locking loop is
subsequently reentered and modulation data is acquired during step
673.
[0065] Returning to FIG. 6, laser assembly 10 may further include
an EEPROM (electrically erasable programmable read-only memory) 83
or other non-volatile storage device for storing information
particular to laser assembly 10. EEPROM 83 may be embodied upon the
same base or within the same housing that includes elements forming
the external cavity laser assembly (e.g., including gain medium
12), and separate from, for example, a printed circuit board upon
which the hardware associated with controller 600 is mounted. Data
may be stored within EEPROM 83 that contains information relevant
to wavelength calibration, tuning hints such as temperatures or
positions, power or temperature calibration factors, identifying
numbers, and operating data. The operating data may contain, for
example, information relevant to laser lifetime, such as
time-current profiles. By storing this information within EEPROM 83
that may be provided as an integral part of laser assembly 10,
interchangeability between laser optic assemblies and controller
boards may be possible while retaining device-specific data.
[0066] In one particular embodiment, calibration coefficients
associated with sensors 631-634 are stored within EEPROM 83. The
calibration coefficients may represent deviations from nominal
values of sensors 631-634. For example, each of the sensors 631-634
may nominally have the same value of a resistance at ambient
temperature. However, due to specific device variations, the actual
values associated with sensors 631-634 may deviate from the nominal
value. The calibration coefficients may represent the relative
differences between the resistances of sensors 631-634 when each is
measured at an equal ambient temperature. These calibration
coefficients may be stored in EEPROM 83 following manufacture of
the laser assembly, and may be used to scale temperature
measurements taken from sensors 631-634, as described further
below.
[0067] The laser controller may further include a network interface
such as an Ethernet interface to allow control of the laser
functionality by a remotely connected device. In one embodiment,
the Ethernet functionality may be used to support an HTTP
interface. Additionally, code for controlling operations of
microprocessor 602 may be upgraded by downloading through an
interface such as, for example, an RS-232 or Ethernet interface.
This functionality allows for on-the-fly upgrades. Similarly, the
logic configuration of FPGA 608 (or any other programmable logic
device) may be modified through an interface such as an RS-232 or
Ethernet interface.
[0068] FIG. 7 illustrates one embodiment of laser current source
626. The laser current source of FIG. 7 is preferably configured to
provide a low noise current to gain medium 12 with reasonable
efficiency. As will be described further below, laser current
source 626 may also include a mechanism to provide a shutdown of
the laser output in the case of a fault condition.
[0069] Current flowing through gain medium 12 is passed through a
transistor 702 and resistor 704. Transistor 702 may be implemented
using a MOSFET (metal oxide semiconductor field effect transistor
device). A filter 706, which may be implemented as an LCR filter,
is provided to filter high frequency noise at a power supply VCC.
Digital-to-analog converter 620 is provided to receive a programmed
value from microprocessor 602 which sets the current flowing
through gain medium 12.
[0070] An operational amplifier 716 regulates the current flowing
through transistor 702 by comparing an output of digital analog
converter 620, which may be passed through an RC filter formed by
resistor 717 and capacitor 719, to a signal at node 721 which is
dependent upon current sensed through transistor 702. The RC filter
formed by resistor 717 and capacitor 719 may provide residual noise
attenuation at mid and upper frequencies. Current flowing through
transistor 702 is sensed in accordance with sense resistor 704 and
an operational amplifier 722. More particularly, operational
amplifier 722 is configured to sense the current flowing through
transistor 702 by measuring the voltage across resistor 704. The
output of operational amplifier 722 is reflected down to a
ground-based voltage using transistor 725, which regulates current
flow through a resistor 723 depending upon the voltage across
resistor 704. Thus, the voltage at node 721 is a ground referenced
voltage indicative of the current flowing through transistor 702.
It is noted that the circuit configuration of FIG. 7 allows one of
the nodes (e.g., the cathode) of gain medium 12 to be grounded.
[0071] The current source of FIG. 7 further includes a transistor
708 connected in a common gate configuration between a node 710 and
the gate of transistor 702. In the embodiment shown, transistor 708
is implemented using a FET (field effect transistor). In other
embodiments, transistor 708 may be implemented using a bi-polar
transistor coupled in a common base configuration. Due to the high
output impedance looking into the drain of transistor 708, lower
frequency noise on the power supply at node 712 is reflected onto
the gate of transistor 702 thus causing VGs to remain constant.
Transistor 708 provides level translation up to the gate of
transistor 702 without introducing a significant power supply
voltage dependence. Thus, while the current flowing through
transistor 702 will be dependent upon the voltage at node 710 which
is controlled by the output of operational amplifier 716, the
output current of transistor 702 is largely unaffected by low
frequency noise on the power supply at node 712. Operational
amplifier 716 maintains the DC current at the programmed level.
[0072] Switch 730, which may be implemented using a transistor such
as a FET or bipolar transistor, provides the user with a fast
acting laser shutdown. Preferably, switch 730 may have a low
voltage threshold so even in a worst-case, a relatively low voltage
may be sufficient to drive the transistor into conduction and
divert the current source from the gain medium 12. It is noted that
control of the switch 730 may be conducted independent of the
operation of microprocessor 602 (FIG. 5). Thus, the laser may be
shut down even if malfunctions associated with the execution of
instructions by microprocessor 602 occur.
[0073] It is noted that in other embodiments, other particular
current source circuits may be employed for providing current to
gain medium 12. Such alternative circuit configurations may employ
a drive transistor for supplying current to a laser device, a
control circuit for controlling the level of current supplied to
the laser device, and a common gate transistor (or common base
transistor) coupled between the control circuit and a control
terminal of the drive transistor to reduce the effects of noise.
Such circuits may additionally employ a switch for diverting
current from the laser device.
[0074] FIG. 8 illustrates one embodiment of analog interface 622
for the measurement of temperatures within laser assembly 10.
Circuit portions that correspond to those of FIG. 6 are numbered
identically. FIG. 9 is a flow diagram depicting a method for
temperature measurements.
[0075] Referring collectively to FIGS. 6, 8 and 9, FPGA 608 sets
multiplexers 651 and 652 in modes to selectively convey a signal
generated by one of the temperature sensors 631-634 or other input
to analog-to-digital converter 624 for data capture within FPGA
608. Additional multiplexer 651 inputs include a ground reference
802 and a precision reference 804. Precision reference input 804
may be implemented using a precision resistor. Depending upon the
mode of multiplexer 651 as controlled by FPGA 608, one input at a
time is coupled to the output of multiplexer 651, which in turn is
coupled to a fixed voltage reference through a fixed resistance
806. Thus, one of the temperature sensors 631-634 or precision
reference 804 may be connected to form the lower leg of voltage
divider. For example, when FPGA 608 sets multiplexer 651 in a mode
that connects temperature sensor 631 to the output of multiplexer
651, current flows from the fixed voltage reference through
resistor 806 and temperature sensor 631, and the voltage at node
808 is measured. The voltage at node 808 is conveyed through a
common measurement path through multiplexer 652 to
analog-to-digital converter 624, where the voltage is converted to
a digital value which may be sampled by FPGA 608, as discussed
previously. FPGA 608 may set multiplexer 651 and 652 to select a
particular one of sensors 631-634, ground reference 802, or
precision reference 804 to take a corresponding measurement.
[0076] Measurements associated with ground reference 802 and
precision reference 804 are performed to allow for the correction
of DC offsets and gain associated with the temperature measurement
circuitry. As illustrated in FIG. 9, in one embodiment, after FPGA
608 has acquired voltage readings associated with all inputs of
multiplexer 651 during step 902, microprocessor 602 may execute
code stored in memory (within RAM 606, for example) to filter the
reference value associated with ground reference 802 and/or
precision reference 804 (step 904) to compute corrected sensor
values (step 906). Subsequently, microprocessor 602 may execute
code to scale the sensor values using calibration coefficients
stored within EEPROM 83 (step 908).
[0077] A lookup table may further be provided within memory (e.g.,
RAM 606) which correlates various corrected voltage readings with
temperature. Thus, during step 910, microprocessor 602 may access
entries within the lookup table to determine a corresponding
temperature associated with each of the temperature sensor
measurements. In one implementation, microprocessor 602 may perform
linear interpolation to increase the resolution of the lookup table
result.
[0078] Turning finally to FIG. 10, a circuit diagram illustrating
one embodiment of an amplifier circuit 619 for generating a
modulation signal to drive electro-optic modulation element 58 is
shown. Circuit portions that correspond to those of FIG. 6 are
numbered identically for simplicity and clarity.
[0079] Referring collectively to FIGS. 6 and 10, FPGA 608 may be
programmed to generate a pulse-width modulated signal at line 617
which is provided to low-pass filter 618. In one particular
implementation, the pulse-width modulated signal is modulated
according to variations which approximate a sinusoidal wave at 20
kHz. The pulse-width modulated signal may be generated according to
a set of stored values within FPGA 608, that are provided by
microprocessor 602. In one particular implementation, a set of 10
values are stored within FPGA 608 to control the particular
modulation associated with the pulse-width modulated signal at line
617.
[0080] Low-pass filter 618 filters the pulse-width modulated signal
at line 617. Thus, a sinusoidal wave form at 20 kHz may be output
from low-pass filter 618. As illustrated in FIG. 10, amplifier
circuit 619 includes a transformer 1004 having a primary connected
in a push-pull configuration (also known as a bridge-tied load).
The amplified modulation signal is inverted by a first operational
amplifier 1006 and again by another operational amplifier 1008.
Thus, the output of operation amplifier 1008 takes the form of a
sinusoidal wave that is 180 degrees out of phase with respect to a
similar sinusoidal signal at the output of operational amplifier
1006.
[0081] It is noted that in other embodiments, other signal
conversion circuits such as other types of analog filters may be
employed in the place of low-pass filter 618 for converting the
digital output of FPGA 608. It is further noted that in other
embodiments, other forms of amplifier circuits may be coupled in a
push-pull configuration to the primary of transformer 1004. For
example, in one embodiment, a class D amplifier may be employed in
the place of low-pass filter 618 and the amplifier circuitry
including operation amplifiers 1006 and 1008. The output of the
class D amplifier may be coupled to drive the primary of
transformer 1004 through an LC filter, and may be coupled in a
push-pull configuration.
[0082] As a result of the push-pull configuration, a voltage of
approximately 2V.sub.CC peak-to-peak variations (twice the supply
voltage) may be generated across the primary of transformer 1004.
Return current through the primary of transformer 1004 is passed
through operational amplifier 1008, rather than running return
current through ground. Noise due to the generation of the 20 kHz
modulation signal on the ground reference may thereby be reduced.
In one embodiment, transformer 1004 has a coil ratio of 120 to 1
thereby generating a voltage of up to 1000 volts peak to peak at
the output of the secondary of the transformer 1004 to drive the
modulation element 58.
[0083] Although the embodiments above have been described in
considerable detail, numerous variations and modifications will
become apparent to those skilled in the art once the above
disclosure is filly appreciated. It is intended that the following
claims be interpreted to embrace all such variations and
modifications.
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